1. Field of the Invention
This invention relates to plastic molding and more particularly thermoforming of both sides of plastic.
2. Description of the Prior Art
To mold both sides of plastic parts basically three methods have been used. These methods are compression molding, injection molding, and thermoforming. In compression molding the plastic is stamped by cooperative mold blocks to achieve the desired contour and shape. As can be appreciated, the cooperative mold blocks must be ruggedly built to withstand the pressures required to properly form the plastic. To shape or pattern both sides of the plastic the mold blocks must also be precision manufactured to properly mate and form the desired surface contour. The requirement of precision and ruggedness results in relatively expensive mold blocks and compression molding machines. Where translucent or transparent plastic parts are desired, compression molding has been found to produce defects in the finished part resulting from the stamping process commonly referred to as squeeze marks. Additionally, the configuration of the part and the shape of the faces of the part are constrained by the draft angle necessary to bring the mold blocks together and to remove the part therefrom.
In injection molding the plastic is heated to a liquid state and is injected into a mold to form the desired part and its surfaces. Again, precision, rugged molds must be provided to receive and hold the liquified plastic usually at high pressure until it solidifies. Accordingly, these molds are expensive. Without the use of expensive articulated tooling, the shape of the part and configuration of the surfaces are constrained by the draft angle required for removal of the completed part from the mold. For example, any ribs, fins or the like can normally only be arranged in the direction which the part is removed from the mold.
One type of thermoforming is exemplified by Stratton, Jr., U.S. Pat. No. 2,973,558 issued Mar. 7, 1961. In conventional thermoforming, a sheet of thermoplastic is heated to temperature sufficiently soft for stretching and forming. A heated assist plug, cushioned by compressed air, displaces the heated plastic into a mold cavity adapted to mold the desired finished part. Evacuating air from the cavity imposes a pressure differential to stretch the heated plastic sheet to conform to the wall of the mold cavity. The plastic cools and hardens in the cavity to the desired shape.
To impose a pattern or mold on one face or side of the plastic, the mold cavity may be suitably designed. However, where both sides require non-complimentary patterns, one side must be prepatterned prior to molding. As is often the case, the heating and molding of the part distorts or destroys the prepattern detracting from the utility of the finished part.